This invention relates to a combustion process having charge stratification for an internal combustion engine having direct injection and spark ignition. In stratified charge operation, charging air is introduced through one of two valve-controlled, optimized-flow intake channels into a cylinder of the internal combustion engine having a roof-like border on the cylinder head side, containing intake valves and at least one exhaust valve and a piston having a piston head adapted to the roof-like border with a recess oriented at the intake end to develop a predominantly swirl flow, with fuel being introduced from an injection nozzle into the swirl flow.
Unexamined German Patent DE 199 18 036 A1 discloses a generic combustion process. In this combustion process in stratified charge operation the charging air is introduced into the cylinder through an intake channel which functions as a filling channel to generate a swirl flow which is essentially concentrated in the rotor recess with the lifting movement of the piston in the direction of top dead center. This rotor recess, which is designed with a circular shape as seen from above, has a trough-like cross section with an extremely deepened circular section from which the trough increases relatively shallowly to the intake side and/or to the side of the injection nozzle, but is designed to rise relatively steeply in relation to the ignition device which is situated at the center relatively. This design pursues the goal of supplying fuel as a rich cloud of fuel to the ignition device, the fuel being introduced in the direction of the recessed section, into the swirl flow through the border of the trough, which rises relatively steeply.
With this known combustion process in stratified charge operation to prevent wall accumulation of the fuel in the rotor recess with the negative result of increased formation of HC and carbon black, accurate control of the fuel stream is required.
The object of this invention is to improve upon a generic combustion process such that a deep rotor recess is avoided in stratified charge operation as the essential device for developing a rich ignitable fuel-air mixture cloud.
This object is achieved with an internal combustion engine having direct injection and spark ignition, whereby in stratified charge operation charging air is introduced through one of two flow-optimized valve-controlled intake channels into a cylinder of the internal combustion engine having a roof-like border containing intake valves and at least one exhaust valve having a piston with a piston head adapted to the roof-like border with a recess oriented to the intake side to create a predominant swirl flow, in which fuel is introduced into the swirl flow from an injector nozzle. Due to the fact that the intake channel, which is operated as a filling channel, is operated essentially unthrottled in stratified charge operation, and this intake channel generates a swirl flow in the area of the rotor recess by means of a relatively steep arrangement of at least 20° in relation to the machine high center plane in the cylinder head, at least from the end of the injection at 60°-30° before the top dead center of the piston, with a tubular swirl core directed essentially in the direction of the cylinder axis and fixed in position in the rotor recess, which is designed to be essentially open on the intake end by means of a rotor recess edge which is up to 0.15 D high and is arranged on the exhaust end at a maximum distance from the center axis of the ignition device of 0.2 D of a cylinder, where D=60-100 mm, such that an ignitable mixture cloud that is compact essentially in the swirl core of the swirl flow is maintained in the area of the ignition device, while the piston continues to approach top dead center until ignition at 0°-35° after the respective end of the injection.
As experimental results have proven, a swirl flow having a tubular swirl core develops in a cylinder having a great charge filling, and this swirl core can be advantageously fixed in position according to this invention by means of a relatively low recess edge on a relatively shallow rotor recess starting at a stroke range of approx. 80° piston position before top dead center with respect to an ignition device arranged in the area of the cylinder axis. According to this invention, fuel is introduced in this swirl core by means of a fuel stream which is controllable as a function of load in its pulse, such that the fuel stream passes only slightly through the swirl core in the direction of the exhaust end. Thus with this invention a compact ignitable mixture cloud is achieved in the area of the ignition device with a reduced risk of wall accumulation on the rotor recess.
In a preferred embodiment of the combustion process according to this invention, fuel is discharged from an injection nozzle arranged beneath the intake channels with a stream axis inclined between 0°=orthogonal to the cylinder axis and 30° on the piston side, the stream axis being inclined downward in the direction of an ignition device that is provided in the area of the cylinder axis, with a pulse that is controllable as a function of load such that the essential part of a spray cone is introduced into the swirl core.
For implementing the combustion process according to this invention, a multihole nozzle or a swirl injector each having a stream cone angle of 40°-55° or 60°-80° or a throttle pin nozzle having a similar stream cone angle is used.
In conjunction with the position of the injection nozzle described above, these values yield the advantage that a section of the fuel tone strikes the ignition device in the swirl cone and thus ensures a reliable start of the combustion.
Using the aforementioned injection nozzles, the combustion process may also be designed so that fuel is discharged from an injection nozzle situated adjacent to the ignition device on the cylinder head end, with a pulse that is controllable as a function of the load, such that a spray cone which introduces the essential fuel component into the swirl core of the swirl flow ends at a predetermined distance from the rotor recess of the piston approaching top dead center.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.